Lean burn is known to give low fuel consumption and low NO.sub.x emissions. There is however a limit at which an engine can be operated with a lean air/fuel mixture because of misfire and combustion instability as a result of slow burn. Known methods to extend the lean limit include improving the ignitability of the mixture by enhancing the fuel preparation, for example using finely atomised fuel or vaporised fuel, and increasing the flame speed by introducing charge motion and turbulence in the air/fuel mixture. Finally, combustion by auto-ignition has been proposed for operating an engine with very lean air/fuel mixtures.
Controlled Auto-ignition Combustion has been given different names according to authors from various research activities world-wide including Homogeneous Charge Compression Ignition (Southwest Research Institute), Premixed Charge Compression Ignition (Toyota and VW), Active Radical Combustion (Honda), Fluid Dynamically Controlled Combustion (French Petroleum Institute), Active Thermo Combustion (Nippon Engines). As the various names imply, when certain conditions are met within a homogeneous charge of lean air/fuel mixture during low load operation, auto-ignition can occur wherein bulk combustion takes place initiated simultaneously from many ignition sites within the charge, resulting in very stable power output, very clean combustion and high thermal efficiency. NO.sub.x emission produced in controlled auto-ignition combustion is extremely low in comparison with spark ignition combustion based on a propagating flame front and heterogeneous charge compression ignition combustion based on an attached diffusion flame. In the latter two cases represented by today's well known spark ignition engine and diesel engine respectively, the burnt gas temperature is highly heterogeneous within the charge with very high local values creating high NO.sub.x emission. By contrast, in controlled auto-ignition combustion where the combustion is uniformly distributed throughout the charge from many ignition sites, the burnt gas temperature is substantially homogeneous with much lower local values resulting in very low NO.sub.x emission.
Engines operating under controlled auto-ignition combustion have already been successfully demonstrated in two-stroke gasoline engines using a conventional compression ratio. It iis believed that the high proportion of burnt gases remaining from the previous cycle, i.e. the residual content, within the two-stroke engine combustion chamber is responsible for providing the hot charge temperature and active fuel radicals necessary to promote auto-ignition in a very lean air/fuel mixture. In four-stroke engines, because the residual content is low, auto-ignition is more difficult to achieve, but can be induced by heating the intake air to a high temperature or by significantly increasing the compression ratio.
In all the above cases, the range of engine speeds and loads in which controlled auto-ignition combustion can be achieved is relatively narrow. The fuel used also has a significant effect on the operating range, for example, diesel fuel and methanol fuel have wider auto-ignition ranges than gasoline fuel.